Trueness of full-arch dental models obtained by digital and conventional impression techniques: an in vivo study

The aim of this study was to compare the trueness of complete- and partial-arch impressions obtained using conventional impression materials and intraoral scanners in vivo. Full-arch impressions were taken using polyether and polyvinylsiloxane. Gypsum casts were digitized using a laboratory scanner (IM, AF). Casts obtained from polyether impressions were also scanned using an industrial blue light scanner to construct 3D reference models. Intraoral scanning was performed using CEREC Omnicam (CO) and Trios 3 (TR). Surface matching software (Atos Professional) enabled to determine the mean deviations (mean distances) from the reference casts. Statistically significant discrepancies were calculated using the Wilcoxon signed-rank test. The mean distance for trueness ranged from 0.005 mm (TR) to 0.023 mm (IM) for the full arch, from 0.001 mm (CO) to 0.068 mm (IM) for the anterior segment, and from 0.019 mm (AF) to 0.042 mm (IM) for the posterior segment. Comparing the anterior vs. the posterior segment, significantly less deviations were observed for anterior with CO (p < 0.001) and TR (p < 0.001). Full-arch comparisons revealed significant differences between AF vs. IM (p = 0.014), IM vs. CO (p = 0.002), and IM vs. TR (p = 0.001). Full-arch trueness was comparable when using Affinis and the two intraoral scanners CEREC Omnicam and Trios 3. The digital impression devices yielded higher local deviations within the complete arch. Digital impressions of the complete arch are a suitable and reliable alternative to conventional impressions. However, they should be used with caution in the posterior region. Trial registration: Registration number at the German Clinical Trial Register (04.02.2022): DRKS00027988 (https://trialsearch.who.int/).

Analysis. All data were stored in STL files for analyses. For three-dimensional difference analysis, surface matching software (Atos Professional; GOM, ZEISS company, Braunschweig, Germany) was used to superimpose the scans according to the best-fit-alignment method. The mean deviation values (mean distance in mm) were obtained by superimposing the respective model for each participant with the associated reference model. This method allows determination of the trueness of different impression methods.
First, the reference model was imported as an STL file, and subsequently trimmed to the tooth shape. The file for comparison was also imported as an STL file. Then, a rough, manual 3-point alignment of the two relevant models had to be implemented via prominent points on the tooth surfaces of the canines and molars for all study participants. Subsequently, the precise adjustment in form of a best-fit alignment could be performed by manual selection of the entire tooth surfaces, enabling the surface matching software to superimpose the models for variance analyses. Surface discrepancies between the two images were represented using color-mapping methods, and numbers. The color maps show a tolerance range of ± 10 µm (green) and a specified range of ± 100 µm (20 color segments). The analysis included all tooth surfaces. The complete arch was considered for evaluation, and a division into anterior and posterior segments was also realised. The total arch included teeth 17 to 27, the anterior segment teeth 13 to 23, and the posterior segment teeth 14 to 17, and 24 to 27. Thereby, the two segments were not scanned separately but were selected from the full-arch dataset of the digitized reference casts. ]) impressions were taken. Then, the reference gypsum cast of the IM-impression was provided with reference points and scanned with ATOS (GOM, ZEISS company) (2). Subsequently, the STL files of the different impression techniques were virtually superimposed on the STL reference model (3) so that the deviations were represented in the form of color-coded distance maps (4).

Results
The mean distance values (± standard deviation) for the complete arch ranged from 0.005 mm (± 0.025 mm) for Trios 3 (TR) to 0.023 mm (± 0.010 mm) for lab scans of the gypsum casts obtained using Impregum (IM). The mean values for the anterior segment ranged from 0.001 mm (± 0.016 mm) for CEREC Omnicam (CO) to 0.068 mm (± 0.146 mm) for IM, and those for the posterior segment ranged from 0.019 mm (± 0.036 mm) for lab scans of the gypsum casts obtained using Affinis (AF) to 0.042 mm (± 0.076 mm) for IM ( Table 1). The mean distances from the reference models were generally similar between the conventional impression method using the silicone material AF and the two intraoral digital scanners. The impressions made using the polyether material IM exhibited the highest discrepancies for all compared areas (Table 1; Fig. 2, 3).
Full-arch comparisons between all impression methods (AF vs. IM, AF vs. CO, AF vs. TR, IM vs. CO, IM vs. TR, and CO vs. TR) revealed significant differences between AF vs. IM (p = 0.014), IM vs. CO (p = 0.002), and IM vs. TR (p = 0.001) (Fig. 2). Specifically, the lab scans of gypsum casts obtained using the polyether impression material (IM) were significantly less precise compared to the silicone impression material (AF) or to both utilized intraoral scanners (CO and TR).
Comparative analysis between the anterior and posterior segment of the dental arch within the same impression group revealed significantly higher values for the posterior segment when using the intraoral digital scanner Omnicam (p < 0.001) or Trios 3 (p < 0.001) (Figs. 3, 4). Thus, with both scanners, the scans of the anterior segment showed significantly less deviations compared to the scans of the posterior segment. The conventional impressions were continuously accurate, with no significant differences between the anterior and posterior areas (p > 0.05).

Discussion
In the present study, the trueness of conventional and digital impression methods for the complete arch was evaluated in vivo. In 31 participants, impressions of the upper jaw were taken with two routinely used conventional impression materials-a polyether and a vinylpolysiloxane material-and two intraoral scanners. The two most routinely materials for high precision impressions were used for conventional impressions [40][41][42][43] . Polyether even gave more accurate impressions than polyvinylsiloxane. It is assumed that this is due to the hydrophilic nature of polyether and its use in the moist intraoral environment [44][45][46] . Assuming that polyether is the most accurate conventional impression material, Impregum Penta Soft was selected for obtaining the reference models. Clinical experience, and previous investigations by other authors, indicate that polyether is a highly accurate material [46][47][48] . Comparison studies have usually used the gypsum cast of a conventional impression as the reference model 12,29,36 . Similar to the study design of Ender et al., the anterior and posterior segments were considered separately for analysis after being digitally cut out of the complete arch 12 . The intention was to test in vivo whether there are differences in the detection of the dental arch between the anterior and posterior areas due to certain influencing factors. The posterior area was previously described as most prone to deviation in the case of intraoral scanning [49][50][51][52] . The method of best-fit alignment was used, as this is the most common direct concept for accuracy evaluations 12,23,24 .
The present results disproved the null hypothesis that there were no statistically significant differences between the conventional and the digital impression techniques for full-arch and partial-arch comparisons.
The accuracy of the impression indisputably determines the fit of the produced prosthetics. There is still no clear evidence how much discrepancy of a dental prosthesis is clinically acceptable. However, many authors propose a clinically acceptable marginal gap of up to 120 µm (incl. 25-50 µm for a cement layer) [53][54][55] . In the present study highly satisfactory mean distance values from 1 µm to 68 µm were obtained (full arch: 5 µm with TR to 23 µm with IM, anterior segment: 1 µm with CO to 68 µm with IM, and posterior segment: 19 µm with AF to 42 µm with IM) 12,29 . The in vitro investigation of Ender et al. showed similar mean deviation values of 16 µm to 93 µm for the complete arch (polyvinylsiloxane vs. Cerec Omnicam v. 4.6.1), 14 µm to 69 µm for the anterior part, and 10 µm to 47 µm for the posterior part (both polyvinylsiloxane vs. Medit i500 v. 1.2.1). They also compared the maxilla, in which they applied various intraoral scanners, and one conventional impression material. The impression with polyvinylsiloxane showed a significant better match with the custom complete-arch reference cast 12 . In contrast, in the present study, the values obtained with AF were generally very similar to those obtained with the digital scanners. Other investigations have reported similar results for conventional impression material and digital devices 30,56 . Especially regarding the complete arch, authors have mainly focused on in vitro studies using representative dental models. Ender et al. concluded that digital impressions could compete with conventional impressions in cases of partial arch capturing 12,30 . However, they and other authors have reported that conventional impression methods still showed higher accuracy when capturing the complete arch 12,27,29,57,58 . This may be largely because digital intraoral scanners only record sections of the dental arch and then match them to a whole image. The software identifies overlapping areas and superimposes them electronically, which may result in inaccuracies 12,28,59 . Nevertheless, some authors have reported that certain devices can keep up with conventional methods for the full arch 30,34,35,60 .
Concerning the fabrication of dental prosthesis, many authors have described intraoral scanning devices as being more precise than conventional impressions when producing single crowns or short-span  Therefore, a meta-analysis concluded that these results should be interpreted with caution 62 . On the other hand, conventional methods are reported to be more precise when producing large or full-arch restorations 58 . To date, there is no general agreement on which impression method is most suitable. The significance analysis revealed that the laboratory scans of gypsum casts made with Impregum (IM) significantly differed from the dental models obtained from impressions made with Affinis (AF) or from the two digital intraoral scanners, CEREC Omnicam (CO) and Trios 3 (TR). Specifically, IM showed significantly less trueness in full-arch comparisons when superimposed with the corresponding digitized reference casts. Better values for IM were expected, since prior studies showed similar accuracy from a polyether and a silicone impression material 63 . However, it can be assumed that the method used to make the reference casts was reliable as the polyether impressions were poured and directly scanned using the high-resolution industrial 3D-scanner Atos Professional. These casts, together with the gypsum casts of the silicone impressions, were digitized using an extraoral laboratory scanner. This could lead to errors, since the casts obtained using the impressions with polyether were transported and scanned twice. It appears that the indirect digitization of stone casts is fraught with errors and may provide inaccurate results 14,23,34,64 . Moreover, by scanning the polyether plaster model with both the high-resolution reference scanner and the extraoral laboratory scanner, the measurement accuracy of the latter scanner should be checked. The superimposition of the two digital models obtained should show maximum match for superimposition. Regarding Fig. 3 statistical outliers in case of IM can be seen at once. Thus, this is due to a measurement inaccuracy of the lab scanner.
The two utilized intraoral scanners showed significant differences when comparing the anterior segment versus posterior segment within the full arch of the same impression group. Superimposition of the anterior segments of the digital intraoral scans with the same fields of the relevant reference models indicated significantly higher trueness compared to the posterior segments. This can be explained by the more difficult conditions for scanning in the posterior area of the jaw, due to the limited oral space for the scan head and the higher condensation from breath or saliva. This finding is in accordance with the observations of Zhang et al.  36,64 . Zhang et al. found no significant differences between the gypsum casts and the digital models generated by the scanner iTero, and concluded that digital impression methods are clinically acceptable, and could replace dental stones 36 . Sfondrini et al. examined 14 participants and determined that the gyspsum casts were not significantly different from the scans using Trios 3. Their conclusions emphasized that intraoral scanning seems to be more time efficient and more comfortable for orthodontic applications, and is on track to replace conventional methods 64 . However, it must be noted that both of these studies used Alginate, which is not a highly precise conventional material 36,64 . Moreover, the poured gypsum casts were not scanned using a high-precision industrial scanner to obtain the reference models, but rather were scanned using the www.nature.com/scientificreports/ same scanner that was employed for the intraoral scans 64 . Ender et al. applied various conventional materials, including polyether, vinylsiloxanether, and Alginate, and employed seven digital scanners on a total of five participants. They found higher precision when using conventional impressions, except for the alginate group. In contrast to the two previously mentioned studies, they found that the scans were more accurate than the alginate impressions 57 . This result was more likely to be expected. Keul and Guth examined the upper jaw of only one participant, using an addition curing silicone as a conventional impression material, and using iTero Element for intraoral scans. A unique aspect of their study was the implementation of an in vivo and an in vitro comparison. For the in vitro part, the silicone impression of the patient was casted with a model resin. Their results from both parts of the study indicated that iTero yielded the same or even higher accuracy for single parameters 34 . Other clinical investigations have focused on single teeth up to segments of the dental arch 7,56,66 .
The limitations of the present study were that no repetitions were conducted. This was due to the in vivo nature of this study, the relatively large number of participants and their compliance, and the associated exposure time. Similarly, impressions are not repeated in the clinical routine if all therapeutically relevant structures have been represented. Thus, in line with the everyday practice, only trueness and not precision was investigated. Some authors have likewise used only one component to describe accuracy 49,[67][68][69][70] . Moreover, to enable an in vivo comparison, an impression method had to be selected for processing the reference models. This was previously established 56 . Therefore, no standardized independent model could be applied, as could be done in an in vitro study. Additionally, the anterior and posterior segments were not scanned separately but were rather generated from the data of the complete-arch scan, as was previously described in another study 12 .

Conclusion
Within the limitations of this in vivo study, scanning the complete dental arch with the tested digital impression systems yielded clinically acceptable results, and represents a suitable and reliable alternative to conventional impressions. The trueness was similar between Affinis and the two intraoral scanners, CEREC Omnicam and Trios 3. Digital impression devices can show higher local deviations within the complete arch. More precise results were obtained for the anterior segment compared to the posterior area. Depending on the clinical problem, the conventional impression can be replaced by the digital impression. Taking into consideration that digital devices should be used with caution in the posterior region.
These results should be verified in additional clinical studies, which should preferably include both jaws and more digital intraoral scanners.

Data availability
The datasets generated and/or analysed during the current study are not publicly available due part of this data from the study will also be used in the upcoming dissertation by Maria Paulig, and it includes patient data but are available from the corresponding author on reasonable request.